u.S. Pat. no. 3,823,739 dated July 16, 1974 is directed to a fail-safe lock-out control valve for a pneumatic actuator and may be manually operated to lock-out a slide valve member until the operating fluid pressure reaches predetermined operating range. When the fluid pressure reaches a high or low pressure outside the operating range, the piston shown in U.S. Pat. No. 3,823,739 moves to a seated position to exhaust fluid from the actuator, and a separate locking member engages the piston to retain the piston positively in seated position to prevent any movement of the piston out of exhaust position until the locking member is manually released. In the event of the malfunctioning or inoperability of the locking member for holding the piston in seated position, vibrations or the like might effect unseating of the piston to a position in which fluid pressure may again be supplied to the actuator for the main valve to move the main valve to an open position. This could result in a possible safety hazard.
Also, in copending application Ser. No. 380,830, of Clifford M. Peters filed July 19, 1973 for "Pressure Controller," an arrangement is disclosed similar to that shown in the above mentioned U.S. Pat. No. 3,823,739 in that a separate manually controlled locking means for the piston is provided so that upon movement of the piston to the down or seated position, the locking means holds the piston in its seated position until the locking means is manually withdrawn. If desired, the locking means may be rotated to an inoperable position out of engagement with the piston and this could result in a safety hazard if the locking means is inadvertently retained in an inoperable position as vibrations or a leakage of fluid in the piston chamber when the piston is in a seated position might move the piston to an up position in which fluid is supplied to the actuator for moving the main flowline valve member to an open position.